10. Spinal cord
The spinal cord is a long cylinder of neural tissue forming the CNS along with the brain. The spinal cord begins at the opening of the foramen magnum and extends down to L1-L2 level. Its function is for the transmission of sensory and motor signals between the brain and the rest of the body, and is a center for coordinating certain reflexes. The spinal cord is enclosed within the spinal canal of the bony vertebral column. Structure: * Extends from the foramen magnum to the conus medullaris near L2, terminating in a fibrous extension known as the filum terminale. * T'wo enlargements:' cervical enlargement (C5-T1) for the brachial plexus and lumbosacral enlargement (L2-S3). These are due to greatly increased mass of motor cells in the anterior horns of the grey matters. * The terminal portion of the spinal cord is called the conus medullaris.The pia mater continues as an extension called the filum terminale, which anchors the spinal cord to the coccyx. The cauda equina ("horse's tail") is the name for the collection of nerves that continue to travel through the vertebral column below the conus medullaris. The cauda equina forms as a result of spinal cord stop growing. Cross-sectional anatomy: The spinal cord consists of a central mass of butterfly or H shaped grey matter enclosed in a cylindrical mass of white matter of motor and sensory neural bundles. It is separated into three sections based on the attachment of the nerve root - anterior/lateral/posterior horn for the grey matter and columns for white matter. It is divided into two halves by the anterior median fissure and posterior median septum. The grey commissure (central limb of the H shaped grey matter), connect the two halves through with the central canal (an extension of the fourth ventricle). A narrow mass of white matter passes anterior to the grey commissure just behind the anterior fissure called white commissure. Grey matter structures: contains cell bodies * Anterior horn: motorneuron cell bodies * Posterior horn: sensory cell bodies * Lateral horn: between segments of T1 to L2, small horn contains preganglionic sympathetic cell bodies. These axons pass out of the anterior nerve roots and enter the spinal nerve from T1 to L2 and into the sympathetic trunk. Between S2-S4, the lateral horn, contains the preganglionic parasympathetic cell bodies. * Laminae: the grey matter are arranged in functional groups with I-X. For example lamina VII include the lateral horn, and laminae IX contains alpha and gamma motoneurons which innervate skeletal muscle. White matter structures: contains axons * Posterior/dorsal column: occupied by the ascending fibres of the gracile and cuneate tracts (pathways for light touch, and sensation). * Anterior and lateral column: The anterior and lateral column are separated by the dividing line of the anterior root respectively. It contains long ascending tract for sensation of temperature, pain, sexual orgasm and cerebellar proprioception and descending tract for motor control. Spinal roots: Total of 31 spinal roots (8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccyx) From the ventral and the dorsal horns/column, rootlets arise and combine to form the ventral and dorsal roots. Upper cervical root are horizontal and lower lumbar pass almost vertically downwards through the subarachnoid space, forming the cauda equina. * The dorsal root contains a ganglion immediately proximal to the point of union of the spinal roots. Cervical ganglion lies outside the intervertebral foramen in contact with the vertebrate artery. * Ventral roots consist of axons from motor neurons, which bring information to the periphery from cell bodies within the CNS. Afferent pathways and ascending tracts: Three destinations, cortex of the opposite cerebral hemisphere via thalamic delay, for conscious sensation, the cerebellum for coordination and brainstem for reflex. These are carried by the white columns (anterior/lateral/posterior). There are three neurons for this pathway, first order from dorsal root ganglion (equivalent to cranial nerve) and ascend the spinal cord, second order within the spinal cord or brainstem (usually decussate to the thalamus), and third from the thalamus via the internal capsule to the cortex * Gracile and cuneate tract - cells bodies in the posterior root ganglia, and enter from the medial side of the posterior root, and travel upwards. Lower fibres are midline, while the incoming fibres are added laterally. Thus lower limb and perineum for gracile, and upper limb form cuneate tract. These end at the gracei and cuneate nuclei and continue as the medial leminiscus. * Anteriolateral tract (spinothalamic tracts) - the bundle lie in front of the level of the attachment of the denticulate ligament. The pain/temperature fibres enter the posterior horn, and through second order neuron bodies in laminae I and V, cross to the opposite side of the cord in front of the central canal and form the anterolateral tract. Fibres from sacrum lie laterally and those higher up are placed more medially. Majority of the tract, synapse with cells of the brainstem reticular formation and subsequently the thalamus. * Anterior/posterior spinocerebellar tract - convey unconscious propriceptive information from cord to cerebellum. Cells from dorsal root ganglion, pass to the dorsal horn synapse to the thoracic nuclei (in thoracic and 2 lumbar segments), the cell axon move to lateral edge of their own side as the posterior spinocerebellar tract. This passes up to the medulla and reach the cerebellar via the inferior peduncle. Other fibres, in the lumbar and sacral segments pass to the opposite side to form the anterior spinocerebellar tract. It take a long route to the mid brain and decussate again to the opposite cerebellar via the superior peduncle. Efferent pathway and descending tracts: Main function is the control of skeletal muscles via spinal nerve. The upper motoneuron pass from the cerebral cortex to the brainstem through the internal capsule and the fibres decussate and end in the anterior horn cells of the cord. The corresponding corticobulbar fibres go to the cranial nerve nuclei. Collectively these form the pyramidal tract as they pass the pyramid of the medulla. The lower motoneuron pass to the skeletal endplate. * Lateral corticospinal: formed from the motor decusssation in the lower medulla. The tract lies in the lateral white column, near the posterior nerve root. * Anterior corticospinal: uncrossed fibres in the medulla and pass no lower than the upper thoracic part of the cord. The tract lies immediately lateral to the anterior median fissure. The fibres decussate at their level of termination. Extrapyramidal tracts: have facilitating roles * Lateral reticulospinal: from medullary part of he reticular formation and pass down in the lateral white column and intermingled with the corticospinal fibres. * Medial reticulospinal: cells in the pontine reticular formation and descending the anterior white column. * Lateral vestibulospinal: arise from lateral vestibular nucleus of the medulla and runs down the cord near the anterior nerve root region. This affects trunk and limb muscular for posture and coordination. * Medial vestibulospinal: much smaller and descending only in the cervical segments and coordinate head movement. Arterial supply''' The spinal cord is supplied by three longitudinal arteries: * single anterior spinal artery: supplies the anterior two-thirds of the spinal cord (anterior and lateral grey/white columns). It is formed at the foramen magnum by the union of two anterior spinal branches of the vertebral artery. * paired posterior spinal arteries: supply the posterior one-third of the spinal cord (the posterior grey/white columns). It is a branch from the posterior inferior cerebellar or vertebral artery above the foramen magnum and runs down the entire length of the cord. * anastomoses between the spinal arteries, called arterial vasocorona, supply the peripheral lateral aspect of the spinal cord (lateral columns). The spinal arteries can be quite discontinuous thus they are reinforcement by segmental (radicular) arteries, which are branches of: * ascending cervical artery * deep cervical artery (costocervical branch) * intercostal arteries * lumbar arteries * sacral arteries These arteries pass through the intervertebral foramen and divide into anterior and posterior radicular arteries and have variable anastomoses with the spinal arteries. The dominant segmental artery is called the artery of Adamkiewicz, which can have a variable origin (lower intercostal or upper lumbar). In the event of arterial supply being interrupted from the aorta (e.g. surgery, trauma) collateral arterial flow can come from the internal thoracic and lateral thoracic arteries, which anastomose with the posterior intercostal arteries. Anastomotic connection on the surface of the cord deep to the pia mater between the arteries are important in supplying peripheral area of the cord. The lateral and anterior corticospinal tract are laminated with sacral paths most superficial, it would be preserved due to pial vessels in the event of loss of anterior spinal supply where the rest of the tract would be injured. Venous drainage Venous drainage largely follows arterial supply. * Anterior and posterior spinal veins * Anterior and posterior radicular veins, which freely communicate with the internal vertebral plexus in the epidural space. This is in turn drains to the cerebral dural venous sinuses and cerebral veins as well as the external vertebral plexus. From the external plexus, segmental veins leave such as vertebral vein in the neck, azygos vein in the thorac and lumbar vein in the lumbar region.